Anyone know how far you can bore the 305, 307 and 327 blocks of the late 60's and early 70"s? Is there a spec book somewhere for each casting number that would give you this info? I tend to see when looking to purchase a block that some are bored +.30. But what is considered a save bore size if that is the correct way to say it for each block.

From what I have been told, it is better to re-sleeve than continue trying to bore. I haven't done either as of yet, boring continues to reduce the thickness of the walls decreasing the strength, I have been told it can cause, if over done, piston slap. Like I stated this is what I have been told. I'm new at this also and have gotten a ton of great information from the people here at CRG!

Blocks cast after the mid-sixties shouldn't be bored more than .060. Any more than that and you're playing with the distinct possibility of a split cylinder wall. The most serious problem will not come from the engineered dimensions but from the fact that the casting process allowed for a range of "core shift" that can result in the cylinders, lifters, etc. to be moved slightly to one side or the other. In stock form they usually don't have a problem but when the boring process opens up the cylinder .060 or more there MAY be instances of cylinder walls thickness in the range of .100. This is not conducive to good ring seal or reliability.

Sleeves in small blocks are also problematic. You may get away with it on street motors but the relatively close bore spacing can cause adjacent cylinders to distort from the pressure required to push the new sleeve into place. If you're trying to salvage a numbers matching block it might be worth the risk but, in general, when things are that far gone it's more cost effective to start over.

From what I have been told, it is better to re-sleeve than continue trying to bore. I haven't done either as of yet, boring continues to reduce the thickness of the walls decreasing the strength, I have been told it can cause, if over done, piston slap. Like I stated this is what I have been told. I'm new at this also and have gotten a ton of great information from the people here at CRG!

dab67

Unless it is a block that has big value - boring past a certain point isn't worth the risk of hitting water or in the case of a daily driver, the over heating / ring seal problems that will likely come with it... Piston slap isn't created by the boring process - its just a function usually, of poorly sizing the newly bored cylinders to the new pistons that would (should) be used in the process. I have no experience with sleeves - but would expect that unless its a big dollar situation (read non-daily driver vehicle) that it would too cost prohibitive for most owners to do, let alone the majority of most machine / engine shops to accomplish properly and well. For the availability of most any small (or big - for that matter) blocks, especially for situations where the vehicle isn't a collector piece requiring matching numbers - anything over a .030" bore would probably prompt me personally to look for another subject block to spend my $$'s on - especially if the goal was to use it for a long time or under severe conditions such as racing... IMHO --Randy

My 67 283 is .120 over on the bore without sleeving it. Its called a four inch over bore. from what i understand very few castings allow this to take place. Most engines will support .90 but it's not recomended you go over .60. due to cylinder thickness.

I've gotta agree with dutch...If you've got to go over .30 on a block to clean it up...then try looking for something else. You'll be money ahead, especially if you looking for something that is going to be used long term. J.R.